To remove carbon-based particulate matter from an exhaust gas discharged from diesel engines, ceramic honeycomb filters having ceramic honeycomb structures with flow paths alternately sealed at both ends has been used. As shown in FIGS. 12(a) and (b), a ceramic honeycomb filter 1 comprises a ceramic honeycomb structure 10 composed of pluralities of cells 13 partitioned by porous cell walls 12, and a peripheral wall 11; and plugs 14a, 14b alternately sealing both end portions 15a, 15b of cells 13 in a checkerboard pattern. The exhaust gas containing particulate matter flows into cells 13aopen on an inlet-side end surface 16a, passes through the cell walls 12, and then flows out of adjacent cells 13b open on an outlet-side end surface 16b, during which particulate matter in the exhaust gas is captured by fine pores (not shown) in the cell walls 12.
To form plugs 14a, 14b in both end portions 15a, 15b of cells 13 of the ceramic honeycomb structure 10, for example, the following method has been conventionally used. That is, sealing films 6 made of a resin, etc. are attached to both end surfaces 16a, 16b of the ceramic honeycomb structure 10 [FIG. 13(a)], and provided with penetrating pores 6a, 6b alternately at positions corresponding to open cells in a checkerboard pattern by laser beams [FIG. 13(b)], such that only one end portion of each cell 13 is sealed by the sealing films 6.
The end surface 16a of the ceramic honeycomb structure 10 is immersed in a plugging material slurry 40 containing ceramic powder and a dispersing medium in a vessel 90 [FIG. 13(c)], and pushed downward to a predetermined depth. The plugging material slurry 40 is introduced into the predetermined cells 13 through the penetrating pores 6a of the sealing film 6, to form plugs 14a of the predetermined length in the end portions 15a of the cells 13 at the end surface 16a [FIG. 13(d)].
The ceramic honeycomb structure 10 provided with the plugs 14a in one-side end portions 15a is taken out of the vessel 90 [FIG. 13(e)]. With the end surface 16b immersed upside down in the plugging material slurry 40, the ceramic honeycomb structure 10 is pushed downward to a predetermined depth, to form plugs 14b in a checkerboard pattern in the other-side end portions 15b of the cells 13 [FIG. 13(f)]. Finally, the ceramic honeycomb structure 10 provided with the plugs 14a, 14b in the end portions 15a, 15b is taken out of the vessel 90 [FIG. 13(g)].
The plugs 14a, 14b are dried and sintered, to obtain the ceramic honeycomb filter 1 having the plugs 14a, 14b in a checkerboard pattern in the end portions 15a, 15b of the cells 13.
In this conventional method, when the ceramic honeycomb structure 10 having a plugging material slurry 40 charged into its end portions is lifted from the vessel 90, the plugging material slurry 40 falls from the end portions, resulting in large unevenness in length of the resultant plugs. A plugging material slurry 40 may drop off from pluralities of cells 13, resulting in a certain area of cells 13 with no plugs. When the plugs in the ceramic honeycomb structure 10 are largely uneven in length, the cell walls filtering an exhaust gas also have largely uneven surface areas, providing a filter having unstable particulate-matter-capturing performance and pressure loss performance, and thus resulting in a defective ceramic honeycomb filter, which should be discarded.
To suppress such unevenness in length of the plugs, it is contemplated to slide a ceramic honeycomb structure 10 having a plugging material slurry 40 charged into its end portions horizontally when taken out of the vessel 90. However, this method not only requires an at least partially detachable vertical wall in the vessel 90, but also fails to conduct the formation of plugs continuously.
As a method for suppressing the unevenness in length of the plugs, JP 2008-55347 A discloses, as shown in FIG. 14, a plugging method comprising (a) using an apparatus comprising a vessel 100 composed of a vertically movable side wall 101 and a bottom 102; a device 110 for holding a ceramic honeycomb structure, which comprises a pressing member 104 attached to a vertically movable arm 103, and a gripping member 105, and a chuck member 120 for sealing a clearance between an opening of the vessel 100 and the gripping member 105 of the gripping device 110; (b) charging a plugging material slurry into the vessel 100; (c) setting the ceramic honeycomb structure 130 gripped by the gripping device 110 in the plugging material slurry in the vessel 100; (d) sealing the clearance of the vessel 100 with the chuck member 120; (e) lifting the bottom of the vessel 100 to introduce the plugging material slurry under pressure into cell end portions of the ceramic honeycomb structure, thereby forming plugs; (f) rotating the bottom 102 of the vessel 100 to separate the plugging material slurry filled in the cell end portions from a small amount of the plugging material slurry remaining on the bottom 102 of the vessel 100, thereby preventing the plugging material slurry in the cell end portions from being pulled back into the vessel 100; (g) moving the side wall 101 downward; and then (h) sliding the ceramic honeycomb structure 130 along the bottom 102 to take it out of the vessel 100.
In the method of JP 2008-55347 A, however, because the bottom 102 of the vessel 100 is rotated while the clearance of the vessel 100 is sealed with the chuck member 120, air does not intrude into the plugging material slurry, failing to easily separate the plugging material slurry even with a shear force. Although the rotation of the bottom 102 gives a shear force to the plugging material slurry, the shear force disappears while the side wall 101 is moved downward, failing to obtain the effect of rotating the ceramic honeycomb structure 130. It has been found that when the ceramic honeycomb structure 130 slides along the bottom 102, the plugging material slurry in the cell end portions is physically separated from that remaining on the bottom 102, making the rotation of the bottom 102 unnecessary.
Furthermore, because the method of JP 2008-55347 A is a batch-type process having many steps, it takes too much time. Particularly, because a batch of a plugging material slurry should be supplied to the vessel 100 after lifting the once-lowered side wall 101, and because a subsequent step should wait until a surface of the plugging material slurry in the vessel 100 becomes flat, large time loss is inevitable. In addition, if the next batch of a plugging material slurry were supplied with some plugging material slurry remaining on the side wall 101 and the bottom 102 of the vessel 100, the amount of the plugging material slurry in the vessel 100 would not be constant, resulting in large unevenness in length of plugs formed in the cells. The supplying of the next batch of a plugging material slurry after removing the plugging material slurry remaining on the side wall 101 and the bottom 102 of the vessel 100 would need additional time.
As a method for forming plugs having uniform length in open end portions of cells, JP 2009-6629 A discloses, as shown in FIG. 15, an apparatus for plugging a honeycomb structure, comprising a plugging material slurry reservoir 201, a plate 202 with pluralities of openings 203 disposed on the reservoir 201, an inlet 204 for supplying a plugging material slurry to the reservoir 201, a valve 205 disposed in the inlet 204, and a piston 206 for supplying the plugging material slurry under pressure through pluralities of openings 203 of the plate 202. Because the plugging apparatus has pluralities of openings 203, the plugging material slurry with high viscosity can pass through the openings 203 under pressure by the piston 206, thereby forming a plugging material slurry layer with uniform thickness on an upper surface of the plate 202. Therefore, a leveling process is not necessary after the plugging material slurry is supplied. By disposing a honeycomb structure 210, to which a sheet having openings corresponding to predetermined cells is attached, on the plugging material slurry layer on an upper surface of the plate 202, and then lifting the piston 206 toward the plate 202, the plugging material slurry is introduced under pressure into the predetermined cells to form plugs. Because the plate 202 has a flat surface, the honeycomb structure 210 can be easily horizontally moved on the plate 202 to be taken out of the plugging apparatus.
Because the honeycomb structure 210 horizontally moves on the plate 202, the apparatus of JP 2009-6629 A does not have such a problem that plugs are pulled back when the honeycomb structure 210 is taken out. However, when the plugging material slurry is introduced under pressure by the piston 206 into the honeycomb structure 210 disposed on the plate 202, the plugging material slurry leaks from a lower edge of an outer periphery of the honeycomb structure 210. JP 2009-6629 A does not describe any mechanism of preventing the leakage of a plugging material slurry. Because the honeycomb structure 210 provided with plugs is horizontally moved on the plate 202 to be taken out of the plugging apparatus, it is not easy to install a mechanism for preventing the leakage of the plugging material slurry on the plate 202. The leakage of a plugging material slurry leads to large unevenness in length of plugs in the resultant honeycomb structures 210, resulting in defective ceramic honeycomb filters, which should be discarded.
As shown in FIG. 16, JP 2004-25098 A discloses a method for producing a ceramic honeycomb filter, comprising the steps of (a) pressing a lower end surface of a ceramic honeycomb structure 301 having flow paths 303 partitioned by pluralities of cells 304 on a bottom surface of a vessel 309 containing a plugging material slurry 308, thereby introducing the plugging material slurry 308 under pressure into predetermined cells 304, to form plugs 302; (b) rotationally lifting the ceramic honeycomb structure 301 by a slight distance, to form an air layer 310 between the plugs 302 and the bottom surface of the vessel 309 to separate them; and then (c) taking the ceramic honeycomb structure 301 provided with the plugs 302 out of the vessel 309. In the method of JP 2004-25098 A, however, because the ceramic honeycomb structure 301 is rotationally lifted from a state where the lower end surface of the ceramic honeycomb structure 301 is in close contact with the bottom surface of the vessel 309, the plugging material slurry 308 left between the ceramic honeycomb structure 301 and the vessel 309 is decompressed, causing the plugs 302 in the cells 304 to be pulled back, resulting in unevenness in their lengths.
As shown in FIG. 17, JP 2008-55796 A discloses a method for plugging a ceramic honeycomb structure 486 by using an apparatus having an annular upper housing 463 communicating with an air inlet, an annular lower housing 464 connected to the upper housing 463, a movable housing 465 disposed in the annular upper housing 463 and the annular lower housing 464, which is inflatable by supplying an compressed air, an elastic body 467 contained in the upper housing 463, which is inflatable by supplying an compressed air, and a second elastic body 466 connected to a vessel; the method comprising gripping a ceramic honeycomb structure 486 with the movable housing 465 and the elastic body 467 inflated by an compressed air; and immersing the ceramic honeycomb structure 486 in slurry stored in the vessel. However, JP 2008-55796 A fails to teach or suggest a process of taking the ceramic honeycomb structure 486 provided with plugs out of the vessel. When the ceramic honeycomb structure 486 provided with plugs is lifted from the vessel without rotation, part of the plugged slurry is pulled back, resulting in unevenness in the length.